Fetus

In amniotic fluid with multiple layers
of protection.

The fetus sits in amniotic fluid, but 1400 years ago nobody knew that the amniotic
fluid is actually water.

The amniotic fluid is the protective liquid contained by the amniotic sac of a gravid amniote. This fluid serves as a cushion for the growing fetus, but also serves to facilitate the exchange of nutrients, water, and biochemical products between mother and fetus.
For humans, the amniotic fluid is commonly called
water or waters (Latin liquor amnii)...

At first, amniotic fluid is mainly
water with electrolytes, but by about the 12-14th week the liquid also contains proteins, carbohydrates, lipids and phospholipids, and urea, all of which aid in the growth of the fetus.
Wikipedia, Amniotic Fluid, 2020

The amniotic fluid for humans is mainly
water. This was known recently however this was
portrayed in the Quran 1400 years before it was
discovered. The Quran describes the amniotic
fluid for humans as water.

[Quran
86.6-7]He was created from
gushing water exiting from between the backbone and the breastbones.

"Ma-e مَاءٍ" in
Arabic means water. The amniotic fluid for humans is
referred to as water.

How could an illiterate man who lived
1400 years ago have known that the amniotic
fluid for humans is actually water?

The fetus is protected by three layers:

Amnio-chorionic membrane.

Uterus.

Abdominal
muscle wall.

A fetus receives from the mother everything
it needs of nutrients, oxygen... A fetus can
even hear sounds however it cannot see
anything because light cannot reach there.
Light is the only thing that doesn't
reach the fetus. This was known
recently, however this was portrayed in the
Quran 1400 years before it was discovered. The Quran says
that the fetus sits inside three darkness's:

[Quran 39.6]He created you from
one person, then made from it its mate, and
brought down cattle for you. Eight pairs He creates you in the wombs of
your mothers, in successive formations, in a
triple darkness. Such is Allah,
your Lord. His is the kingdom. There is no
god but He. So what made you deviate?

A fetus sits in complete darkness inside
three protective layers.

How could an illiterate man who lived
1400 years ago have known that light is the only
thing that doesn't reach the fetus?

To Christians

[Quran 42.14-15]
They only disbanded after they received knowledge (of their Bible) fallaciously among them. And if it were not a word from your Lord [Allah] that postponed it to a later date, it would have been all settled among them then. And those who inherited the Book (the Bible) from them are in great suspicion of it. Therefore call them (to the Quran) and straighten your ways as you are ordered and don't follow their passions and say: "I believe in what Allah revealed of the Book (The Quran) and I was ordered to be just among you (both Christians and Jews); Allah is our Lord and your Lord, we have our actions and you have your actions, no argument between us and you, Allah brings us together and to Him we are (all) destined."

To Muslims

[Quran 6.108]
Do not curse those who call (to worship) other than Allah, so that they don't curse Allah out of spite and ignorance. This is how We [Allah] appealed to each people its own actions; then they will be brought back to their Lord [Allah] who will inform them of what they have been doing.